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  • What is target poisoning in magnetron sputtering? What are the general influencing factors?
    Oct 24, 2018

    What is target poisoning in magnetron sputtering? What are the general influencing factors?

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    First: the formation of target surface metal compounds.

    Where is the compound formed from the metal target through reactive sputtering? As reactive gas particles collide with target atoms to produce chemical reactions to form compound atoms, which are usually exothermic reactions, the reaction formation heat must have a way of conducting out, otherwise, the chemical reaction cannot continue. Heat conduction between gases is impossible under vacuum conditions, so chemical reactions must take place on a solid surface. Reactive sputtering products are performed on target surface, substrate surface, and other structural surfaces. The generation of compounds on the substrate surface is our goal. The generation of compounds on other structural surfaces is a waste of resources. The generation of compounds on the target surface is initially a source of compound atoms, but later becomes an obstacle to the continuous supply of more compound atoms.

     

    Second, the influencing factors of target poisoning

    Target poisoning is mainly affected by the proportion of reactive gas and sputtering gas. In the process of reactive sputtering, the sputtering channel area on the target surface is covered by reactive products or the reactive products are stripped and the metal surface is re-exposed. If the formation rate of the compound is greater than the rate at which the compound is stripped, the compound coverage area increases. In the case of a certain power, the amount of reactive gases involved in the formation of the compound increases, and the rate of formation of the compound increases. If the reactive gas quantity increases excessively and the compound coverage area increases, if the reactive gas flow cannot be timely adjusted, the rate of the increase of the compound coverage area will not be inhibited, and the sputtering channel will be further covered by the compound. When the sputtering target is completely covered by the compound, the target will be completely poisoned.

     

    Third: target poisoning phenomenon

    (1) positive ion accumulation: when the target is poisoned, an insulating film is formed on the target surface. When the positive ions reach the cathode target surface, they cannot enter the cathode target surface directly due to the barrier of the insulation layer. Instead, they are deposited on the target surface, which is likely to produce arc discharge --- arc striking in the cold field, making the cathode sputtering unable to proceed. (2) anodic disappearance: when the target is poisoned, insulating film is deposited on the vacuum chamber wall of the ground, and electrons reaching the anode cannot enter the anode, forming anodic disappearance phenomenon.

     

    Fourth: physical explanation of target poisoning

    (1) in general, the secondary electron emission coefficient of metal compounds is higher than that of metal. After target poisoning, the surface of target material is all metal compounds. After being bombarded by ions, the number of secondary electrons released increases, which improves the permeability of space, reduces the resistance of plasma and leads to the decrease of sputtering voltage. The sputtering rate is reduced. Generally, the sputtering voltage of magnetron sputtering is between 400V and 600V. When target poisoning occurs, the sputtering voltage will be significantly reduced. (2) the sputtering rate of metal target is different from that of compound target. Generally, the sputtering coefficient of metal is higher than that of compound, so the sputtering rate is lower after target poisoning. (3) the sputtering efficiency of reactive sputtering gas is lower than that of inert gas, so the integrated sputtering rate decreases after the proportion of reactive gas increases.

     

    Fifth: the solution of target poisoning

    (1) medium frequency power or rf power.

    (2) closed loop control is adopted to control the intake quantity of reactive gas.

    (3) adopt twin targets

    (4) control the change of coating mode: the hysteresis effect curve of target poisoning is collected before coating, so that the intake flow is controlled at the front of target poisoning, and the process is always in the mode before the deposition rate drops sharply.

     image

    It is relatively easy to sputter metal atoms on the surface of the target. Generally requires radio frequency sputtering.

    Ion bombardment makes metal atoms on the surface of the target very lively. At this time, the target surface is undergoing both sputtering and reaction to form compounds. If the sputtering rate is greater than the compound generation rate, the target is in the metal sputter state. Conversely, if the reaction gas pressure increases or the metal sputtering rate decreases, the target may suddenly have a compound formation rate exceeding the sputtering rate and stop sputtering.

    In order to reduce target poisoning, technicians often use the following methods :(1) send reactive gas and sputtering gas respectively to the substrate and near the target to form pressure gradient; (2) increase the exhaust rate; (3) gas pulse introduction; (4) plasma monitoring, etc.

     

    Target poisoning is caused by the accumulation of positive ions on the surface of the target in the process of sputtering, which is not neutralized. As a result, negative bias pressure on the surface of the target gradually decreases. Finally, the target poisoning is simply stopped working.

     

    The main cause of target poisoning is that the synthesis speed of the medium is greater than the sputtering yield (too much oxidation reaction gas is injected), resulting in the loss of conductive ability of the conductor target. Only when the breakdown voltage is improved can the target shine, the voltage is too high and arc discharge occurs easily. Phenomenon: the target voltage cannot reach normal for a long time, and it is always running at low voltage, accompanied by arc discharge. The surface of the target shows white adherents or heavily pin-shaped gray discharge traces. If target poisoning is to be eliminated, medium frequency power supply or rf power supply should be used instead of dc power supply. Reducing the intake amount of reactive gas, increasing sputtering power, cleaning contaminants on target materials (especially oil pollution), and selecting dust and arc extinguishing cover with good vacuum performance can effectively prevent target poisoning. The magnet soaked in the cooling water inside the target material has stains. As long as the magnetic field strength is sufficient and the cooling effect is good, it has little influence on the target material.

     

    Besmirch effect is not big ~ strike fire is to have insulation place to cause, it is local poison commonly or dirt. Target poisoning is due to the low power density, and relative to the excess reactive gas cannot be evaporated (or sprayed) in time, which will result in the residual surface of the target, resulting in decreased electrical conductivity, thus entering the toxic state. Light cannot glow, heavy scrap power.


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